Mo-Cu composite powder

ABSTRACT

A Mo—Cu composite powder is provided which is comprised of individual finite particles each having a copper phase and a molybdenum phase wherein the molybdenum phase substantially encapsulates the copper phase. The composite powder may be consolidated by conventional P/M techniques and sintered without copper bleedout according to the method described herein to produce Mo—Cu pseudoalloy articles having very good shape retention, a high sintered density, and a fine microstructure.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/193,023, filed Mar. 29, 2000.

TECHNICAL FIELD

This invention relates to molybdenum-copper (Mo—Cu) composite powdersand in particular to Mo—Cu composite powders used to make components forelectronics, electronic packaging, and electrical engineeringapplications. Examples of such applications include heat sinks, thermalspreaders, electrical contacts, and welding electrodes.

BACKGROUND ART

Mo—Cu pseudoalloys possess properties that are similar to the propertiesof W—Cu pseudoalloys. However, they have the additional advantages oflower weight and higher workability which makes them better suited forminiaturized electronics.

One conventional method for making articles comprised of Mo—Cupseudoalloys consists of infiltrating separately sintered porousmolybdenum blanks with liquid copper. Infiltrated articles have a solidmolybdenum skeleton that functions as the backbone of the pseudoalloy.The skeleton retains the liquid copper during infiltration (and hightemperature operation) by capillary forces. One drawback of theinfiltration method is that it does not allow near-net or net-shapefabrication of parts. Hence, a number of machining operations arerequired to obtain the final shape of the infiltrated article.

Other conventional methods for forming Mo—Cu articles includeconsolidating blends of molybdenum and copper powders by powdermetallurgical (P/M) techniques such as hot pressing, explosive pressing,injection molding, tape forming, and rolling. Unlike the infiltrationmethod, these methods do not have a separate step for sintering amolybdenum skeleton. As a result, articles made by P/M methods eithercompletely lack a molybdenum skeleton or have a skeleton of reducedstrength. High compacting pressure, repressing, resintering, andsintering under pressure (hot pressing) have been suggested to improveMo—Mo contacts and the strength of the Mo skeleton. Although P/Mtechniques allow near-net or net-shape fabrication, sintering articlesto full density is complicated by the lack of solubility in the Mo—Cusystem, poor wetting of molybdenum by copper, and by copper bleedoutfrom parts during sintering. Furthermore, additions of sinteringactivators such as nickel and cobalt to improve densification aredetrimental to the thermal conductivity of Mo—Cu pseudoalloys, aproperty which is critical for a number of electronics applications.

In order to improve the homogeneity and density of Mo—Cu pseudoalloysmade by P/M methods, Mo—Cu composite powders have been used wherein themolybdenum particles have been coated with copper by chemical depositionor electroplating. However, the copper coating reduces the contact areabetween molybdenum particles and the strength of the molybdenumskeleton. Moreover, these powders do not prevent copper bleedout fromparts during sintering, and hot pressing is still required to improvethe sintered density of articles. Thus, it would be advantageous to havea Mo—Cu composite powder which could be used in P/M methods to form netor near-net shaped Mo—Cu articles having strong sintered molybdenumskeletons without copper bleedout.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is another object of the invention to provide a Mo—Cu compositepowder with a phase distribution that facilitates the formation of astrong molybdenum skeleton and internal infiltration of the skeletonwith liquid copper during sintering.

It is a further object of the invention to provide a Mo—Cu compositeoxide powder for producing a Mo—Cu composite powder having a high levelof mixing of the metal phases.

It is still a further object of the invention to provide a P/M method ofmaking Mo—Cu pseudoalloy articles with a strong molybdenum skeleton anda high sintered density without copper bleedout.

In accordance with an object of the invention, there is provided amolybdenum-copper composite powder comprising individual finiteparticles each having a copper phase and a molybdenum phase wherein themolybdenum phase substantially encapsulates the copper phase.

In accordance with another object of the invention, there is provided amethod of making a CuMoO₄-based composite oxide powder comprising:

(a) forming a mixture of a molybdenum oxide and a copper oxide, themolybdenum oxide being selected from ammonium dimolybdate, ammoniumparamolybdate, or molybdenum dioxide; and

(b) firing the mixture at a temperature and for a time sufficient toform the CuMoO₄-based composite oxide.

In accordance with still another object of the invention, the Mo—Cucomposite powder of this invention is made by the method comprising:

(a) reducing a CuMoO₄-based composite oxide powder in a first stage toform an intimate mixture of metallic copper and molybdenum oxideswithout the formation of low-melting-point cuprous molybdate phases; and

(b) reducing the intimate mixture in a second stage at a temperature andfor a time sufficient to reduce the molybdenum oxides to molybdenummetal.

In another aspect of the invention, there is provided a method formaking a Mo—Cu pseudoalloy comprising:

(a) consolidating a Mo—Cu composite powder to form a compact, the Mo—Cucomposite powder having a copper content from about 2 wt. % to about 40wt. % and comprising individual finite particles each having a copperphase and a molybdenum phase wherein the molybdenum phase substantiallyencapsulates the copper phase;

(b) sintering the compact in a first sintering stage at a temperaturefrom about 1030° C. to about 1050° C. to form a molybdenum skeleton;

(c) sintering the compact in a second sintering stage at a temperaturefrom about 1050° C. to about 1080° C. for a compact made from acomposite powder having a copper content of about 26 wt. % to about 40wt. %, or at a temperature from about 1085° C. to about 1400° C. for acompact made from a composite powder having a copper content of about 2wt. % to about 25 wt. %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an x-ray diffraction pattern of a first stage reductionproduct formed by the hydrogen reduction at 300° C. of a CuMoO₄-basedcomposite oxide powder having a relative copper content of 15 wt. %.

FIG. 2 is an SEM micrograph of a cross section of an agglomerate of theMo—Cu composite powder taken using Back-Scattered Electron Imaging.

FIG. 3 is an enlargement of the finite particle outlined in themicrograph shown in FIG. 2.

FIG. 4 is an SEM micrograph of a cross section of a Mo-15Cu pseudoalloy.

FIG. 5 is an SEM micrograph of a cross section of a Mo-40Cu pseudoalloy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

We have invented a molybdenum-copper (Mo—Cu) composite powder whichcomprises finite dual-phase particles each having a copper phase and amolybdenum phase wherein the copper phase is substantially encapsulatedby the molybdenum phase. The bulk Mo—Cu composite powders of thisinvention have the gray color of unalloyed molybdenum powders which isconsistent with the substantial encapsulation of the copper phase by themolybdenum phase. Preferably, the Mo—Cu composite powders contain fromabout 2 wt. % to about 40 wt. % Cu.

In general, the as-reduced Mo—Cu composite powders consist of largeragglomerates (on the order of about 15 μm to about 25 μm in size) of thefinite dual-phase particles. Under SEM examination, the finite particleswithin the agglomerates are shown to be irregularly shaped and have asize in the range of about 0.5 μm to about 1.5 μm. Each of the finiteparticles has a sintered molybdenum network in which the voids arefilled with copper. This unique distribution of Mo and Cu phasesprovides substantial encapsulation of the Cu phase by the Mo phase andresults in the highest level of mixing within the larger agglomerates.

Because the copper phase is encapsulated by the molybdenum phase, anenhanced sintering process with several desirable features is achieved.These features include: (1) the formation of Mo—Mo particle contactsafter pressing the powder, (2) the sintering of a substantially dense Moskeleton prior to melting of the copper, (3) internal infiltration ofthe skeleton with liquid copper and retention of copper within theskeleton by capillary forces, and (4) sintering in the presence ofliquid copper without copper bleedout from the compact.

The Mo—Cu composite powders are produced by the chemical synthesis andhydrogen reduction of cupric molybdate-based composite oxide systemshaving controlled amounts of molybdenum trioxide (MoO₃). Generally,cupric molybdate (CuMoO₄) is made by a solid phase reaction between MoO₃and CuO or Cu₂O in air at 600° C. for 40 hours. However, the coppermetal content of CuMoO₄ as a percentage the total metal content (Cu+Mo)is quite high, about 40 weight percent (wt. %). This is much higher thanthe copper content in Mo—Cu pseudoalloys used in some industrialapplications. For instance, a copper content of 15 to 25 wt. % isrequired in Mo—Cu materials for electronic packaging. This problem issolved by transitioning the copper content of the CuMoO₄-based compositeoxide over a wide range by co-synthesis of varied amounts of a secondphase of MoO₃. The combination of the CuMoO₄ and MoO₃ phases lowers thecopper content of the composite oxide into the ranges desired forelectronics applications. Preferably, the copper content of theCuMoO₄-based composite oxide as a percentage of the total metal contentmay be varied from about 2 wt. % to about 40 wt. %. Unless otherwiseindicated, the relative copper content for the composite oxides isexpressed herein as a percentage of the total metal content of theoxide.

CuMoO₄-based Composite Oxide Synthesis

The composite oxides were produced by solid phase synthesis. The ratioof solid reactants (copper and molybdenum oxides) was adequate tosynthesize end products containing the CuMoO₄ phase and a controlledamount of the MoO₃ phase. The ratio of the synthesized phases(particularly, the amount of MoO₃) controlled the copper content in theCuMoO₄-based composite oxides. The general formula of the preferredCuMoO₄-based composite oxide may be represented on a mole basis asCuMoO₄+xMoO₃ where x is from about 29 to 0. The co-synthesized CuMoO₄and MoO₃ phases were present in every composite oxide particle whichprovided a very high level of mixing of the copper and molybdenum.

In the preferred synthesis methods, two combinations of reactants areused: (i) complex molybdenum oxides with copper oxides, in particular,ammonium dimolybdate (ADM, (NH₄)₂Mo₂O₇) or ammonium paramolybdate (APM,(NH₄)₆(Mo₇O_(24.)4H₂O) with cuprous (Cu₂O) or cupric (CuO) oxides, and(ii) molybdenum dioxide (MoO₂) with cuprous or cupric oxide. Attemperatures above 250° C. in air, the complex molybdenum oxides undergothermal decomposition (e.g., (NH₄)₂Mo₂O₇→2MoO₃+2NH₃+H₂O), and Cu₂O andMoO₂ undergo oxidation to CuO and MoO₃. These phase transitionsdramatically increase the surface area and surface energy of thereactants which accelerates their solid phase interdiffusion reactionsand the formation of CuMoO₄-based composite oxides. Thus, theCuMoO₄-based composite oxide may be formed by firing a mixture of theseoxides at a temperature from about 650° C. to about 750° C. for onlyabout 5 hours.

Silica was selected as the material of choice to contain the solid phasesynthesis of the CuMoO₄-based composite oxides because molybdenumtrioxide wants to react and form molybdates with the majority of othermetals and metal oxides traditionally used to make boats and trays forsolid phase synthesis processes. The use of silica boats slightlyincreased the silica content of the composite oxide compared to thetotal silica content in the reactants. However, the total silicaremained at a low level and is not believed to substantially affect thesinterability of the final Mo—Cu composite powder or theelectrical/thermal conductivity of the Mo—Cu pseudoalloy.

The first preferred synthesis method may be illustrated by the reactioninvolving ADM and Cu₂O. The composition of ADM can be represented as(NH₄)₂O.2MoO₃, and the synthesis reaction as:0.5Cu₂O+n[(NH₄)₂O.2MoO₃)]+0.25O₂→CuMoO₄+(2n−1)MoO₃+2 nNH₃ +nH₂O

By varying the factor n in the range of about 15.0 to 0.5, the relativecopper content in the CuMoO₄-based composite oxides synthesized by theabove reaction may be controlled in the range of about 2 wt. % to about40 wt. %.

EXAMPLE 1

ADM with a median particle size of 198.8 μm and cuprous oxide with amedian particle size of 14.5 μm were used as solid reactants in thesynthesis of CuMoO₄-based composite oxides. (Unless otherwise specifiedparticle sizes were determined using a Microtrac MT, X-100 particle sizeanalyzer.) The total weight of solid reactants in these tests was in therange of 0.5 to 1.0 kg. Blends were prepared in an alumina ball mill.The weight ratio of alumina milling media to the reactants varied in therange of 1.5 to 1.0. The length of milling was 1 hour. The light browncolor of the milled blend of oxides was the result of mixing the colorsof ADM (white) and Cu₂O (brown). The use of milling is preferred withthese reactants because the median particle size of the ADM and APMpowders is generally much larger than that of the copper oxides whichmakes it difficult to obtain a homogenous blend by mechanical mixingalone.

Synthesis was carried out in air in a laboratory furnace with an aluminatube. Silica boats were used as reaction containers. A load of 150 g ofthe milled oxides produced a bed depth of about 0.5″ in the boat. Therate of the furnace temperature increase was 2° C./min. The synthesistemperature was 750° C. with an isothermal hold at this temperature of 5hours. Five tests were carried out in which the relative copper contentin the synthesized composite oxide was varied in the range of 8 to 40wt. %.

All of the synthesized composite oxides formed sintered cakes whichneeded to be ground up in a mortar. The caking was attributed to thehigh reaction temperature and high diffusion activity of the milledsolid reactants, particularly, the in-situ-produced MoO₃. The groundmaterials were screened −100 mesh. The synthesized powders had agreen-yellow color characteristic of the CuMoO₄-based composite oxides.Elemental and x-ray diffraction (XRD) phase analysis were used forpowder characterization. The major XRD peaks associated with the MoO₃(3.26 Å) and CuMoO₄ (3.73 Å) phases were used to calculate the XRD peakintensity ratios. Table 1 compares the calculated values for thecomposite oxides based on the amounts of the reactants with the measuredvalues for the synthesized composite oxides.

TABLE 1 Calculated Values Measured Values Relative Relative XRD PeakCopper Molar Ratio of Copper Intensity Ratio Content, wt. % MoO₃/CuMoO₄Content, wt. % of MoO₃/CuMoO₄ 8 6.616 7.6 5.23 16 2.477 15.5 1.48 241.097 23.8 0.94 32 0.404 31.7 0.38 40 0 39.9 0.15

The correlation between the measured and the calculated values of therelative copper content increased with the copper content of thesynthesized powders and was in the range of 95.0% to 99.75%. A goodcorrelation was also observed between the trends for the predicted andthe actual ratios of the product phases versus the copper content.

In the second and more preferred synthesis method, a friableCuMoO₄-based composite oxide is made from unmilled dehydrated solidreactants, of which at least one reactant undergoes a phase change inthe course of synthesis, e.g., in-situ oxidation of MoO₂ to MoO₃ andCu₂O to CuO. As in the first method, the copper content of thesynthesized composite oxide is controlled by varying the reactionstoichiometry. The synthesis reaction uses one mole of CuO or a halfmole of Cu₂O. For Cu₂O, the reaction may be represented as:0.5Cu₂O+nMoO₂+(0.5n+0.25)O₂→CuMoO₄+(n−1)MoO₃

By varying the factor n in the range of about 30.0 to 1.0, the relativecopper content in the CuMoO₄-based composite oxides may be controlled inthe range of about 2 wt. % to about 40 wt. %. Also, since the medianparticle size of the molybdenum dioxide and copper oxides is of the sameorder of magnitude, homogeneous starting blends of the solid reactantscould be made by mechanical mixing without milling.

EXAMPLE 2

In this example, MoO₂ (D₅₀=5.3 μm), Cu₂O (D₅₀=14.5 μm), and CuO(D₅₀=13.3 μm) were used to make the composite oxides. Starting blendswith a total weight of 0.5 to 1.0 kg were made by mixing the oxides in alaboratory V-blender for 30 min. The color of the starting blends rangedfrom brown (MoO₂+Cu₂O) to dark brown (MoO₂+CuO).

The synthesis was carried out in air using the same hardware as inExample 1. A load of 100 g of the blended oxides produced a bed depth ofthe material in the boat of about 0.5 inches. The rate of furnacetemperature increase was 2° C./min. The synthesis temperature was 650°C. with an isothermal hold of 5 hours at this temperature. The relativecopper content in the synthesized composite oxide was varied in therange of 5 to 40 wt. %.

In each case, a uniform loosely sintered cake was formed. The materialwas very friable, and could be disintegrated into powder by rubbinglightly between one's fingers. The synthesized powders had agreen-yellow color characteristic of the CuMoO₄-based composite oxides.Powders were screened −100 mesh and subjected to the same analysesdescribed in Example 1. The data in Table 2 illustrate the properties ofthe composite oxide powders synthesized from MoO₂ and Cu₂O (column A)and MoO₂ and CuO (column B).

TABLE 2 Calculated Values Measured Values Relative XRD Peak CopperRelative Copper Intensity Ratio Content, Molar Ratio of Content, wt. %of MoO₃/CuMoO₄ wt. % MoO₃/CuMoO₄ A B A B 5 11.583 4.68 4.66 7.2 7.2 104.961 9.64 9.60 3.2 4.3 15 2.753 14.45 14.47 1.9 2.3 20 1.649 19.3019.49 1.2 1.8 25 0.987 24.39 24.27 0.8 0.9 30 0.545 29.52 29.62 0.5 0.335 0.230 34.53 34.66 0.1 0.3 40 0 39.40 39.78 0 0.1

The correlation between the measured and the calculated values of therelative copper content increased with the copper content and was in therange of 93.4% to 99.0%. A good correlation is also observed between thetrends for the predicted and the actual ratios of the product phasesversus the copper content.

EXAMPLE 3

Additional tests were carried out to demonstrate that the copper contentin the CuMoO₄-based composite oxides can be adjusted to the specifiedlevel by controlling the amount of the copper oxide participating in thesynthesis. An excess of 4 wt. % Cu₂O, compared to the quantity requiredby the reaction stoichiometry, was used. Test conditions were exactlythe same as in Example 2. Table 3 compares the actual copper content ofthe synthesized composite oxide with the copper content specified bystoichiometry. The correlation of the actual copper content with thespecified values was calculated as a ratio of the actual to thespecified copper content.

TABLE 3 Specified Actual (No Actual (4 wt. % Corre- Cu, wt. % ExcessCu₂O) Correlation Excess Cu₂O) lation 5 4.68 0.936 5.08 1.016 10 9.640.964 10.39 1.039 15 14.45 0.963 15.39 1.026 20 19.30 0.965 19.63 0.98125 24.39 0.975 24.97 0.998 30 29.52 0.984 29.75 0.991 35 34.53 0.98635.76 1.022 40 39.40 0.985 40.53 1.013

These results demonstrate that adding an excess of up to 4 wt. % of thecopper oxide reactant over that required by stoichiometry can be used toadjust the relative copper content in the resultant composite oxidecloser to the specified level.

EXAMPLE 4

The synthesis of a CuMoO₄-based composite oxide with a relative coppercontent of 15 wt. % was performed in a production scale belt furnace.The solid reactants were MoO₂ and Cu₂O. A 300 kg blend of reactants witha 4 wt. % excess of Cu₂O was made in a production scale V-blender. A 1.5kg amount of the starting blend produced a material bed depth of about0.5 inches in a silica tray. The synthesis was carried out in air at anaverage temperature of 675° C. with an average residence time of about 4hours. The furnace throughput was about 6 kg of composite oxide perhour. A total of 268 kg of the end product was synthesized. The materialwas discharged from the silica tray onto a vibrating screen,disintegrated, screened −60 mesh and collected in a hopper. The productwas blended in a V-blender and analyzed for particle size distributionand copper content. A sample of the end product was milled, screened−100 mesh and subjected to an XRD analysis. The following productcharacteristics were obtained:

Particle size distribution: D₉₀ = 18.5 μm D₅₀ = 5.5 μm D₁₀ = 2.1 μmRelative copper content: 15.36 wt. % XRD Peak intensity ratio ofMoO₃/CuMoO₄: 1.8

The phase composition and copper content of the composite oxide powdersynthesized in the production furnace closely reproduced thecorresponding properties of powders synthesized in the laboratory.

Reduction of CuMoO₄-based Composite Oxides

One of the key problems which exists in the conventional methodsinvolving the co-reduction of mechanically blended mixtures of oxidepowders stems from the significant difference between the reductiontemperatures of the oxides of molybdenum and copper. This differencecauses a premature appearance of copper and its segregation bycoalescence. This in turn results in an inhomogeneous distribution ofthe Mo and Cu phases in the reduced Mo—Cu composite powders. Incontrast, the Mo—Cu composite powders produced by the hydrogen reductionof the synthesized CuMoO₄-based composite oxides exhibit very goodhomogeneity. The atomic level of contact between the copper andmolybdenum in the synthesized CuMoO₄-based composite oxides and thedifference in the reduction temperatures can be used to advantage bycontrolling the order of appearance of the metal phases therebyresulting in a homogeneous Mo—Cu composite metal powder comprised ofindividual dual-phase particles in which the Mo phase substantiallyencapsulates the Cu phase.

In a preferred method, the hydrogen reduction of the CuMoO₄-basedcomposite oxides is performed in two stages. The first stage reductionis performed at a temperature from about 250° C. to about 400° C. andcauses the reduction of copper from the composite oxides therebyyielding an intimate mixture of metallic copper and molybdenum oxides.The second stage reduction is performed at a higher temperature, fromabout 700° C. to about 950° C., and causes the reduction of themolybdenum oxides to molybdenum metal which results in the formation ofthe dual-phase particles and the substantial encapsulation of the copperphase by the molybdenum phase.

The two-stage reduction is preferred because the hydrogen reduction ofthe CuMoO₄ phase is complicated by disproportionation of cupricmolybdate into cuprous molybdates, Cu₆Mo₄O₁₅ and Cu₂Mo₃O₁₀, which haverelatively low melting points (466° C. and 532° C., respectively). Theformation of these phases at the initial stage of hydrogen reduction isdetrimental as it fuses the powder and obstructs the reduction process.It was discovered that the formation of these liquid phases can beprevented by taking advantage of the high thermodynamic probability thatthe copper in the CuMoO₄-based composite oxides may be reduced attemperatures below the melting points of the cuprous molybdates. Usingthe lower reduction temperatures in a first-stage reduction eliminatesthe formation of the low-melting-point cuprous molybdates and producesan intimate mixture of Mo oxides and metallic copper. Although traces ofother molybdates, Cu₃Mo₂O₉ and Cu₆Mo₅O₁₈, have been identified asforming in the course of the first stage hydrogen reduction, thesemolybdates have a high temperature stability and do not cause anycomplications.

In the second stage, the molybdenum oxides are reduced to molybdenummetal. The conventional method for reducing Mo from its trioxidetypically involves two steps which are carried out in differenttemperature ranges. First, MoO₃ is reduced to MoO₂ at 600–700° C. andthen the MoO₂ is reduced to Mo at 950–1100° C. However, in the reductionof CuMoO₄-based oxide composites, there appears to be a catalytic effectcaused by the freshly reduced Cu phase being in close contact with theMo oxides. This results in a lowering of the temperature of theMoO₃→MoO₂ reduction step to 350–400° C., and the MoO₂→Mo reduction stepto 700–950° C. In addition, the presence of the Cu phase leads to thedeposition of Mo on Cu which inhibits the coalescence and growth ofcopper particles and causes the gradual encapsulation of the Cu phase bythe Mo phase. This mechanism is believed to contribute to controllingthe size and homogeneity of the composite Mo—Cu particles.

After the second reduction stage, the as-reduced Mo—Cu composite powdersmay require passivation to reduce their tendency toward oxidation andpyrophoricity. In particular, it was discovered that oxidation andpyrophoricity of the as-reduced Mo—Cu composite powders with an oxygencontent below 5000 ppm may be suppressed by passivation of the powdersfor 1 to 2 hours with nitrogen immediately after removal from thefurnace.

The following examples illustrate the reduction of the synthesizedCuMoO₄-based composite oxides to form the Mo—Cu composite powders ofthis invention.

EXAMPLE 5

A first stage reduction of a CuMoO₄-based composite oxide having arelative copper content of 15 wt. % was carried out in a laboratoryfurnace with flowing hydrogen. An oxide load of about 150 g produced amaterial bed depth of about 0.5 inches in the Inconel boat. Thereduction temperatures were 150, 200, 300, and 400° C. The rate offurnace temperature increase was 5° C./min, and the isothermal hold atthe reduction temperature was 4 hours. The resulting products werescreened −60 mesh and subjected to an XRD analysis.

No low-melting-point cuprous molybdates (Cu₆Mo₄O₁₅ or Cu₂Mo₃O₁₀) weredetected in the reduction products. Minor Cu₃Mo₂O₉ and Cu₆Mo₅O₁₈ phaseswere detected in products reduced in the 150–200° C. temperature range.The reduction of copper appears to begin at about 200° C. and iscomplete at about 300° C. The major phases in the material reduced at300° C. were Cu, MoO₂, and MoO₃. Apparently, underreduced MoO₃ in thematerial is quite active and undergoes an exothermic, partial hydrationwhen exposed to air. This required that the product reduced at 300° C.be cooled in air for about 20 to 30 min. Once cooled, the reducedproduct which had a dark gray color could be easily screened −60 mesh.The major phases of the product reduced at 400° C. were Cu and MoO₂. Inthis case, the product temperature did not increase after exposure toair. The product reduced at 400° C. was caked and requireddisintegration to turn it into a powder that could be screened −60 mesh.

EXAMPLE 6

A first stage reduction of the same CuMoO₄-based composite oxide used inExample 5 was performed in a large production-scale hydrogen reductionfurnace having three heating zones. An oxide load of about 2 kg produceda material bed depth of about 0.5 inches in an Inconel tray. In thefirst test, all zone temperatures were set at 300° C. In the secondtest, all zone temperatures were set at 400° C. The residence time forthe material in the furnace was about 4 hours. The end products werescreened −60 mesh and subjected to an XRD analysis.

After removal from the furnace, the temperature of the product from thefirst test increased requiring cooling in air for about 30–45 min. Themajor phases in the reduced product were Cu and MoO₂. The minor phasesconsisted of various hydrated molybdenum trioxide phases. FIG. 1illustrates the XRD pattern for this material. The appearance of theproduct closely resembled the material obtained in Example 5 at 300° C.The major phases in the reduced product from the second test were Cu andMoO₂. Traces of the hydrated molybdenum trioxide phases were alsopresent. The appearance of the product also closely resembled thematerial obtained in Example 5 at 400° C.

EXAMPLE 7

A two-stage hydrogen reduction of the synthesized CuMoO₄-based compositeoxides having a relative copper content in the range of 5 to 40 wt. %was performed. The same hardware, loading conditions, and rate oftemperature increase as in Example 5 were used. The reductiontemperatures were 300° C. (first stage) and 700° C. (second stage) witha four-hour isothermal hold at each temperature. After cooling thefurnace to below 200° C., the gas flow through the furnace tube wasswitched from hydrogen to nitrogen. The nitrogen flow was maintaineduntil the furnace cooled to about 30° C. This effectively passivated thereduced Mo—Cu composite metal powders. The bulk as-reduced powders had agray color similar to unalloyed Mo powders. There was no visualindication of the presence of copper in the Mo—Cu composite powders. Thepowders were screened −60 mesh and analyzed for Cu content, particlesize distribution, and surface area. The results of the differentanalyses are given in Table 4.

TABLE 4 Mo—Cu Composite Powder Properties Cu Median Surface CuMoO₄-basedestimated, Cu actual, Size, Area, Composite Oxide wt. % wt. % μm m²/gCuMoO₄ + 11.583 MoO₃ 5 4.9 18.5 2.13 CuMoO₄ + 4.961 MoO₃ 10 10.2 19.71.53 CuMoO₄ + 2.753 MoO₃ 15 15.3 15.7 1.95 CuMoO₄ + 1.649 MoO₃ 20 20.423.5 1.61 CuMoO₄ + 0.987 MoO₃ 25 25.9 22.3 (2.06) CuMoO₄ + 0.545 MoO₃ 3030.9 23.7 1.19 CuMoO₄ + 0.230 MoO₃ 35 36.2 25.1 1.44 CuMoO₄ ~40 41.425.4 1.23

The measured copper content of the Mo—Cu composite powders was generallyabout 2% to about 3.5% higher than the estimated value.

EXAMPLE 8

Tests were conducted on establishing the effect of the second-stagereduction temperature on properties of the resultant Mo—Cu compositepowders. The starting material was the CuMoO₄-based composite oxide (15wt. % Cu) which had been reduced at 300° C. in Example 6. Thesecond-stage reduction was carried out using temperatures in the rangeof 700° C. to 950° C. using the same conditions as in Example 7 exceptthe rate of temperature increase which was 20° C./min. Six reductionruns were carried out. The reduced Mo—Cu composite powders were screened−60 mesh and analyzed. The results of the analyses are presented inTable 5.

TABLE 5 Reduction Temperature, ° C. Powder Property 700 750 800 850 900950 Particle Size Distribution, μm D₉₀ 45.4 47.7 58.0 52.9 49.2 44.5 D₅₀18.5 20.0 21.9 20.3 17.2 16.4 D₁₀  3.2  3.6  4.2  3.2  3.1  3.3 FisherSub-Sieve Size,  2.7  3.1  3.2  3.1  2.9  (3.5) μm Bulk density, g/cm³ 1.15  1.16  1.13  1.10  1.10  1.06 Oxygen Content, ppm 3400 4500 11001100 1050 980 Specific Surface Area,  4.97  4.08  1.89  1.06  0.7  0.57m²/g Calculated Particle  0.13  0.16  0.34  0.61  0.93  1.13 Diameter,μm

As a rule, powder agglomeration due to sintering increases with thereduction temperature. The test results demonstrate that, within a broadrange of reduction temperatures, the size of the reduced Mo—Cu compositepowder and its bulk density do not increase monotonically withtemperature. Furthermore, there appears to be a pronounced sinteringeffect with increased temperature which manifests itself in a decreasein the specific surface area and, correspondingly, an increase of thecalculated particle diameter (the BET particle size). Similarly, theoxygen content follows the trend of the surface area and decreases withtemperature.

The bulk as-reduced Mo—Cu composite powders had a gray color similar tounalloyed Mo powders. There was no visual indication of the presence ofCu in the Mo—Cu composite powders. In order to examine the distributionof phases in the composite powders, they were analyzed by SputteredNeutral Mass Spectrometry (SNMS) and cross-sectioned metallographicsamples were analyzed by Scanning Electron Microscopy (SEM) usingSecondary Electron Imaging (SEI) and Back-Scattered Electron Imaging(BEI).

The Mo—Cu composite powders of this invention were shown to consist oflarger agglomerates of finite dual-phase particles comprised of asintered molybdenum network wherein the voids in the network are filledwith copper. This unique distribution of phases resulted in thesubstantial encapsulation of the copper phase by the molybdenum phase.As shown in the SEM micrographs, the finite particles were irregularlyshaped and ranged in size from about 0.5 to about 1.5 μm. This is inrelative agreement with the particle size calculated from the BETsurface area. FIG. 3 (BEI) is an enlarged image of the finite particleoutlined in the agglomerate shown in FIG. 2 (BEI) and demonstrates theencapsulation of the copper phase by the molybdenum phase. The SNMS testresults are consistent with the SEM observations in that they showdepletion of copper at the surface of the composite powder particles anda very homogeneous distribution of phases within the powder.

EXAMPLE 9

Production scale tests were carried out using a two-stage reduction of asynthesized CuMoO₄-based composite oxide having a relative coppercontent of 15 wt. %. The first stage reduction was done in the samefurnace as in Example 6 using the same loading conditions and areduction temperature of 300° C. The end product was screened −60 meshand subjected to the second-stage reduction in a three-zone hydrogenreduction furnace having a temperature of 900° C. in all three zones. Anoxide load of about 300 g produced a bed depth of about ½″ in an Inconelboat. The residence time for the material in the hot zone of the furnacewas about 4 hours. After removal from the furnace, the reduced powderwas immediately dumped for surface passivation into a stainless steelhopper having a nitrogen atmosphere. Surface passivation for 1 to 2hours completely eliminated the pyrophoricity of the powder. Theresulting Mo—Cu composite powder was screened −60 mesh and shown to havethe following properties:

Particle Size Distribution: D₉₀ = 47.0 μm D₅₀ = 17.9 μm D₁₀ = 2.8 μmFisher Sub-Sieve Size 2.9 μm Specific Surface Area: 1.65 m²/g CalculatedParticle Diameter: 0.39 μm Oxygen Content: 2700 ppm Bulk Density: 1.26g/cm³ Copper Content: 15.06 wt. %

The size of the powder made in this Example and the powder made at 900°C. in Example 8 are very similar. However, the surface area and oxygencontent are substantially higher while the BET particle size issubstantially lower. This indicates that the finite particles formed inthis Example are smaller than those formed in Example 8.

Consolidation of the Mo—Cu Composite Powders

The formation of a rigid Mo skeleton during solid-state sintering isbeneficial to obtaining good dimensional stability of Mo—Cu pseudoalloyparts made by P/M. Mo—Cu pseudoalloys with a strong Mo skeleton resistdistortion during densification even in the presence of large amounts ofliquid copper. High dimensional tolerances and an absence of distortionare of particular importance for the P/M net-shape manufacturing ofthermal management components (heat sinks) for microelectronic andoptoelectronic applications.

In contrast to the Mo—Cu composite powders of this invention, thesintering of mechanical blends of elemental Mo and Cu powders issluggish. High sintering temperatures (up to 1650–1670° C.) are requiredto sinter the P/M compacts from blended metal powders which lead to theloss of copper in the form of bleedout and evaporation from the parts.The loss of copper makes it very difficult to achieve sintered densitiesabove 97% of theoretical density (TD). The use of sintering aids (Fe,Co, Ni) to improve the sinterability of such elemental powder blends ishighly undesirable as the thermal conductivity of Mo—Cu pseudoalloys isdramatically reduced.

For the Mo—Cu composite powders of this invention, we found that thecopper content and distribution of Mo and Cu phases strongly influencedthe sintering conditions of the powder compacts. An inverse relationshipwas observed between the copper content and the sintering temperature ofthe compact.

In particular, sintering temperatures were found to extend from thesolid-state sintering region of 1050–1080° C. for compacts having acopper content in the range of 26–40 wt. % to the region of sintering inthe presence of liquid copper at 1085–1400° C. for compacts having acopper content in the range of 2–25 wt. %.

Sintering in the presence of liquid copper included two steps whichmimic the conventional infiltration method, viz., in-situ sintering of amolybdenum skeleton and internal infiltration of the skeleton withliquid copper. Upon the melting of copper at 1083° C., the molybdenumskeleton is internally infiltrated with liquid copper via capillaryinfiltration. The liquid copper is retained within the molybdenumskeleton by capillary pressure. Dissolved oxygen is removed from themolten copper at 1085–1100° C. The molybdenum skeleton is furthersintered in the presence of the liquid copper to complete thedensification of the pseudoalloy.

The Mo—Cu composite powders may be consolidated in as-reduced,deagglomerated, or spray-dried flowable states. A lubricant and/orbinder may be mixed with the powder, or added during spray drying, toenhance powder consolidation. These materials may include for examplezinc stearate, ethylene-bis-stearamide, or ethylene glycol. The Mo—Cucomposite powders may be used in a number of conventional P/Mconsolidation methods such as mechanical or isostatic pressing,injection molding, tape forming, rolling, and screen printing forceramic metallization.

The following are the preferred processing steps for dewaxing andsintering green compacts made from the Mo—Cu composite powders of thisinvention:

-   -   1. Depending on the type of wax/binder, dewax/debind the green        compacts at a temperature from about 200° C. to about 450° C.;    -   2. Remove oxygen from the green compacts at a temperature from        about 930° C. to about 960° C.;    -   3. Sinter a substantially dense molybdenum skeleton at a        temperature from about 1030° C. to about 1050° C.;    -   4(a). Solid-state sinter compacts with a copper content of about        26 wt. % to about 40 wt. % at a temperature from about 1050° C.        to about 1080° C.; or    -   4(b). Sinter compacts with a copper content of about 2 wt. % to        about 25 wt. % in the presence of a liquid phase at a        temperature from about 1085° C. to about 1400° C.

The Mo—Cu pseudoalloy shapes produced according to this method exhibitedno copper bleedout, very good shape retention, a high sintered density(about 97% to about 99% TD), and a fine pseudoalloy microstructure (Mograins in the range of about 1 μm to about 5 μm; copper pools in therange of about 2 μm to about 15 μm).

EXAMPLE 10

Mo-15Cu pseudoalloy samples were made from the Mo—Cu composite powder(15 wt. % Cu) made in Example 9. To enhance consolidation, the powderwas blended with 0.5 wt. % ethylene-bis-stearamide, a solid lubricantmade under the trade name of Acrawax C by Lonza, Inc. in Fair Lawn, N.J.The powder was mechanically pressed at 70 ksi into flat samples(33.78×33.78×1.62 mm) having a green density of about 62% TD. To assureuniform heat transfer to samples during dewaxing and sintering, thesamples were processed in pure alumina sand. Thermal processing was donein flowing hydrogen in a laboratory furnace with an alumina tube. Toprevent cracking of the tube by thermal stresses, the heating/coolingrate was limited to 2° C./min. The sintering cycle included: 1-hourisothermal holds at 450 and 950° C. for removing the powder lubricantand surface oxygen; a 1-hour isothermal hold at 1040° C. for in-situsintering of a molybdenum skeleton; a 2-hour isothermal hold at 1100° C.for internal infiltration of the skeleton with liquid copper, removal ofdissolved oxygen from molten copper, and presintering the samples; and a2-hour isothermal hold at 1230° C. for final densification of thesamples. The latter temperature was experimentally determined on thebasis of obtaining the highest pseudoalloy density without causingcopper bleedout by oversintering the molybdenum skeleton.

In several consecutive runs (3 samples per run), the as-reduced Mo-15Cupowder demonstrated very good sinterability, an absence of copperbleedout, and good shape retention of the sintered compacts. The averagelinear shrinkage was 15%, and the average values of the sintered densityand electrical conductivity were in the range of, correspondingly,98.8–99.0% TD and 36.6–36.7% IACS.

The thermal conductivity of the sintered samples was determined fromreported correlations between the electrical and thermal conductivity inMo—Cu pseudoalloys. For an infiltrated Mo-15Cu pseudoalloy, anelectrical resistivity of 51.0 nΩ.m (equivalent to an electricalconductivity of 33.8% IACS) corresponds to a thermal conductivity of 166W/m·K. A measured 1.085X increase in electrical conductivity for thesamples made from the Mo-15Cu composite powder raised the thermalconductivity of the samples to a substantially higher level of about 180W/m·K.

An SEM micrograph of a cross section of one of the Mo-15Cu pseudoalloysamples is shown in FIG. 4. The molybdenum skeleton of the pseudoalloyis formed by mostly rounded, highly interconnected grains whosedistribution, order and size have been affected by regrouping andlimited growth in the presence of the liquid phase. The size of thegrains is in the range of about 1 to about 5 microns. Roundedinterconnected grains are indicative of a sintering mechanism consistingof particle rearrangement in the presence of a liquid phase and grainshape accommodation aided by the minute solubility of molybdenum inliquid copper at the sintering temperature. The average size of the Cupools is in the range of about 2 to about 15 microns. Deagglomeration ofthe as-reduced powder before sintering is expected to substantiallyimprove the microstructural homogeneity of the P/M pseudoalloy.

EXAMPLE 11

Mo-40Cu pseudoalloy samples were made from the Mo—Cu composite powder(40 wt. % Cu) made in Example 7. Samples were pressed using the sameconditions as in Example 10. The higher copper content substantiallyimproved the pressibility of samples which exhibited a green density of73% TD. As in Example 10, the temperature for the final densificationwas experimentally determined on the basis of obtaining the highestpseudoalloy density without causing copper bleedout by oversintering themolybdenum skeleton. It was established that the high copper contentlimited the final densification temperature to 1065° C. thus bringing itinto the solid-state sintering region.

In two consecutive runs (3 samples per run), the as-reduced Mo-40Cupowder demonstrated very good sinterability and shape retention of thesintered compacts. The average linear shrinkage was 9%, and the averagevalues of the sintered density and electrical conductivity were in therange of, correspondingly, 97.8–97.9% TD and 50.7–51.0% IACS. The lowerlinear shrinkage compared to that for Mo-15Cu samples in Example 10 canbe explained by the fact that the Mo-40Cu samples were pressed to ahigher green density and consolidated to a lower sintered density.

An SEM micrograph of a cross section of a Mo-40Cu pseudoalloy sample isshown in FIG. 5. By comparing the micrographs in FIGS. 4 and 5, adramatic difference between the solid-state sintering and sintering inthe presence of liquid phase becomes evident. The molybdenum skeleton,that has been sintered in-situ at 1040° C., has only slightly changedduring sintering at 1065° C. The clusters of Mo particles, whose sizeand geometry have been barely affected by sintering, are indicative ofthe absence of the particle rearrangement and the size accommodationsintering mechanisms that are operational only in the presence of aliquid phase. Correspondingly, the microstructure of the solid-statesintered pseudoalloy is less orderly (more clusters of Mo particles,larger Cu pools) than the microstructure of the pseudoalloy sintered inthe presence of the liquid phase. However, the high sintered density ofthe solid-state sintered material indicates that deagglomeration of theas-reduced Mo-40Cu powder before sintering may substantially improve themicrostructural homogeneity of the P/M pseudoalloy.

While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

1. A molybdenum-copper composite powder comprising individual finiteparticles each having a copper phase and a molybdenum phase, theindividual finite particles further having a sintered molybdenum networkwherein the voids in the network are filled with copper and themolybdenum phase substantially encapsulates the copper phase.
 2. Thecomposite powder of claim 1 wherein the individual particles have a sizeof about 0.5 μm to about 1.5 μm.
 3. The composite powder of claim 2wherein the composite powder comprises agglomerates of the finiteparticles.
 4. The composite powder of claim 3 wherein the agglomerateshave a size of about 15 μm to about 25 μm.
 5. The composite powder ofclaim 1 wherein the powder contains from about 2 wt. % to about 40 wt. %copper.
 6. The composite powder of claim 1, wherein the powder has thecolor of unalloyed molybdenum powder.
 7. A method of making a Mo—Cucomposite powder comprising: (a) reducing a CuMoO₄-based composite oxidepowder in a first stage to form an intimate mixture of metallic copperand molybdenum oxides without the formation of low-melting-point cuprousmolybdate phases; and (b) reducing the intimate mixture in a secondstage at a temperature and for a time sufficient to reduce themolybdenum oxides to molybdenum metal.
 8. The method of claim 7 whereinthe first stage reduction is performed at a temperature from about 250°C. to about 400° C.
 9. The method of claim 8 wherein the second stagereduction is performed at a temperature from about 700° C. to about 950°C.
 10. The method of claim 7 wherein the low-melting-point cuprousmolybdate phases are Cu₆Mo₄O₁₅ and Cu₂Mo₃O₁₀.
 11. The method of claim 7wherein the Mo—Cu composite powder is passivated in nitrogen after thesecond stage reduction.